1. Field of the Invention
This invention relates to image data processing. More particularly, this invention relates to error concealment within the field of image data processing.
2. Description of the Prior Art
Known digital video tape recording systems include apparatus and methods for performing error concealment as an integral part of the overall operation. Given the high information densities and stringent requirements with which these systems must cope, errors frequently occur in the reading of the data from the media on which it is stored.
As a first level of protection against such errors, error correction codes are included within the data structure. If the error is a minor one then it is often possible to uniquely identify the particular piece or pieces of information that are in error and what data they should be replaced with. If the error is too severe, then such error correction cannot cope and then reliance is made upon error concealment techniques to reduce the perceivable effect of the error.
In order to facilitate error concealment, the image data is sub sampled into a number of different data processing and recording channels. In this way, if an error occurs in one channel, then there will be data from the remaining channels surrounding the missing data points from the defective channel. For each erroneous pixel, a replacement pixel value can be interpolated from the surrounding pixel values from the other channels. Whilst detail is still lost from the image by such errors, the overall of such error concealment effect is to make the error less immediately perceivable.
In view of the high information densities involved in image data processing, particularly as image definition increases, it is desirable that some form of data compression be performed upon the image data before it is recorded. One set of techniques for achieving such data compression involve the transformation of the image data from the spatial domain into the spatial frequency domain. Once transformed into the spatial frequency domain, the redundancy within the image data can be better exploited to yield efficient compression. The data is stored or transmitted as an encoded version of the image in the spatial frequency domain.
One approach to error concealment within the spatial frequency domain is to adopt an analogous approach to that described above, i.e. data is to sub sampled it into a number of channels then any erroneous parts in the reproduced spatial frequency domain data are interpolated from the immediately adjacent parts of the spatial frequency domain data.
It has been found that the performance of such an error concealment strategy can be poor. In particular, if the image data is transformed into the spatial frequency domain using discrete cosine transformation techniques operating on fixed size blocks of pixels, then an error in a cosine coefficient in the spatial frequency domain will have an effect upon all of the pixels within the corresponding block of pixels in the spatial domain. Since the interpolation in any domain only produces an approximate replacement, the missing cosine coefficient will be likely to be replaced by a value slightly different from the original. The effect once the interpolated replacement has been transformed to the spatial domain is to make the edges of the blocks of pixels used by the transformation technique become visible due to the introduction of discontinuities in the image values between adjacent blocks. The appearance of such regular geometric features within an image is particularly noticeable.
It is an object of the invention to provide improved error concealment within systems processing image data and utilizing domain transformation between the spatial and spatial frequency domains.